1. Technical Field
The present invention relates to an electric power steering control system for an automobile and to a control method thereof, in which an assist force when manipulating a steering wheel is generated by means of a motor.
2. Background Art
As a power steering system for an automobile, an electric power steering apparatus has been developed which apparatus detects a vehicle speed and a steering torque acting on a steering shaft, drives a motor by a drive current determined in response to the detected vehicle speed and torque, adds an assist force onto the steering shaft by means of a rotational force of the motor, and then provides a driver a comfortable steering feeling.
Hitherto, in such a type of electric power steering apparatus, as a motor for adding the assist force onto the steering shaft, a DC electric motor with a brush such as a commutator motor has been mainly used. Since the motor is with a brush, however, it is possible that any trouble or abnormal state occurs in the motor itself.
Therefore, recently use of a brushless motor has been proposed instead of the DC electric motor with a brush. However, such a motor and/or a control apparatus come to be complicated and expensive. Thus, to put into practice the control suitable for the power steering apparatus, several inherent problems to be overcome remain in terms of cost or performance.
In the electric power steering control system, the motor for assisting the steering is connected to the steering wheel through a reduction gear. As he driver directly touches the steering wheel, the motor is required to have less fluctuation in torque and less vibration. Moreover, As rotation of the motor follows a steering speed of the steering wheel, the motor is forced to abruptly start or stop depending upon steering conditions.
Considering an arrangement of the control system of the electric power steering control unit using a brushless motor, as motor current of the brushless motor is an alternating current, the current control may get worse due to the steering conditions depending upon the arrangement of the current control system. As a result, an error in torque control and transient vibration are increased and the steering assist performance of the electric power steering control system is lowered in some cases.
For example, FIG. 4 shows an arrangement of an electric power steering control system which a PM (permanent magnet) brushless motor is used as a steering assist motor and the electric current is controlled with a three-phase alternating current.
Referring to FIG. 4, a PM brushless motor 5a is PWM driven by a PWM drive section 101. A control computer 200 computes three-phase voltage command values Vu*, Vv*, Vw* on the basis of a vehicle speed detected by a speed sensor 6, a steering torque detected by a torque sensor 3, a rotational position of the brushless motor 5 detected by a position sensor 103, and three-phase alternating currents Iu, Iv detected by current sensors 102a, 102b. Then the PWM drive 101 is controlled by the mentioned three-phase voltage command values Vu*, Vv*, Vw*.
The control computer 200 includes a d-q axis current command section 200a for computing d-axis and q-axis current command values Id*, Iq* for generating a steering assist force in response to a predetermined characteristic on the basis of the steering torque and the vehicle speed, an electrical angle computing section 200b for computing an electrical angle signal xcex8 in response to the rotational position of the brushless motor 5 detected by the position sensor 103, a d-q coordinate inverse transformation section 200c for performing d-q coordinate inverse transformation of the d-axis and q-axis current command values Id*, Iq* on the basis of the electrical angle signal xcex8 thereby computing three-phase current command values Iu*, Iv*, and a three-phase voltage command section 200d for computing the three-phase voltage command values Vu*, Vv*, Vw* in accordance with deviation values of the three-phase current command values Iu*, Iv* and the three.-phase alternating currents Iu, Iv inputted to the brushless motor 5.
The PM brushless motor 5 generates a torque by delivering sine-wave currents to the three phases in accordance with the electrical angle as shown in FIG. 6, for example. As the motor output torque is in proportion to amplitude of the sine-wave currents, controlling the current amplitude controls the output torque of the PM brushless motor 5.
The mentioned electrical angle is an angle in which pairs of S and N poles of a rotor in the PM brushless motor are arranged to be 360 degrees. By way of example, FIG. 5 shows the relation between electrical angle and mechanical angle in case of an eight-pole motor.
In the electric power steering control system as described above, under the condition of holding the steering wheel, generating a torque without rotating the brushless motor 5 performs steering assist. For example, supposing that the electrical angle is positioned at 50 [deg] (on a broken line in FIG. 6), the electric current flowing in the form of three phases become a direct current as shown in FIG. 7. Therefore the current command value is coincident with the actual electric current without deterioration in current control characteristic, and the steering assist characteristic is not deteriorated.
Under the condition of rotating the steering wheel, as the brushless motor 5 rotates, it is required that the three-phase current depicts sine waves as described above. Under the condition of relatively gentle steering such as in the case of driving the vehicle at a high speed, the current control system can follow up the steering condition and the steering assist characteristic is not deteriorated.
However, in the case that the brushless motor 5 rotates at a high speed, there is a possibility that, in the control by means of the three-phase alternating currents, a phase shift is generated between the current command value and the actual electric current depending on the current control characteristic.
More specifically, in the electric power steering control system, the rotation speed of the brushless motor 5 follows up the rotation speed of the steering wheel, and therefore when the steering wheel is turned sharply, the motor is abruptly rotated to a speed over the rotation speed performance thereof. As the motor is connected to the steering wheel through the reduction gear (for example, at gear ratio 28), frequency of the sine-wave current flowing into the motor becomes as follows. That is, gear ratio times (28 times) the steering rotation frequencyxc3x97pole pair number times (4 times in case of the eight-pole motor)=112 times. Accordingly, frequency of the three-phase alternating current applied to the brushless motor 5 rises abruptly, and therefore in a system in which the electric current is controlled by the three-phase alternating current as shown in FIG. 4, the current control cannot follow up such a sharp rise and a phase shift takes place between the current command value and the actual electric current.
In the PM brushless motor, the phase shift means increase in reactive current, and supposing a phase shift with respect to an ideal current phase as xcex1, the output torque is in proportion to COS xcex1. In other words, when any phase shift is generated, the output torque of the steering assist motor is lowered, and as a result, manipulation of the steering wheel becomes heavy.
In the electric power steering control system, it is essential that the steering torque does not increase even when the steering wheel is turned sharply in terms of emergency behavior to avoid accident, and it is important to arrange the electric power steering control system so as not to increase the steering torque even when the steering wheel is turned sharply.
The present invention was made to solve the above-discussed problems and has an object of providing an electric power steering control system in which a brushless motor is used as a motor for generating a steering assist force, and error in torque control and transient vibration are small even when the steering wheel is turned sharply. The invention also provides a method for controlling electric power steering.
An electric power steering control system according to the invention arranged to control a drive current of a brushless motor that generates a steering assist force on the basis of a steering torque acting on a steering shaft and a vehicle speed, comprises:
a d-q-axis current command section for computing a d- and q-axis current command value for generating a steering assist force in response to a predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed;
an electrical angle computing section for computing an electrical angle signal on the basis of a rotational position signal of the mentioned brushless motor;
a d-q coordinate transformation section for performing a d-q coordinate transformation on the basis of a detection current signal in response to a three-phase AC current to be inputted to the mentioned brushless motor and of the mentioned electrical angle signal, thereby obtaining a d- and q-axis detection current value;
a d-q axis voltage command section for computing a d- and q-axis voltage command value in response to a deviation between the mentioned d- and q-axis current command value and the mentioned d- and q-axis detection current value;
a d-q coordinate inverse transformation section for performing a d-q coordinate inverse transformation on the basis of the mentioned d- and q-axis voltage command value and the mentioned electrical angle signal thereby computing a three-phase voltage command value; and
a PWM output section for generating a PWM waveform for PWM drive of the mentioned brushless motor in response to the mentioned three-phase voltage command value.
Thus, the system is arranged so that the brushless motor is used as the motor for generating a steering assist force, and the current control is performed on the d-q coordinate axis where the electric current can be treated in dc amount even when the motor is driven, and as a result, it is possible to achieve a reliable electric power steering control system in which error in torque control and transient vibration are both small even when the steering wheel is turned sharply.
It is preferable that the mentioned d-q axis current command section includes a q-axis current command part for computing the q-axis current command value for generating the steering assist force in response to a predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed and a d-axis current setting part for setting zero as the d-axis current command value.
It is preferable that a computing expression for performing the d-q coordinate transformation on the basis of the mentioned detected current signal and the mentioned electrical angle signal is stored in the mentioned d-q coordinate transformation section.
It is preferable that a following mathematical expression is used as the mentioned computing expression:       [                            Id                                      Iq                      ]    =                    2            ⁡              [                                                                              -                  sin                                ⁢                                  xe2x80x83                                ⁢                                  (                                      θ                    +                                                                  4                        3                                            ⁢                                              xe2x80x83                                            ⁢                      π                                                        )                                                                                    sin                ⁢                                  xe2x80x83                                ⁢                θ                                                                                                          -                  sin                                ⁢                                  xe2x80x83                                ⁢                                  (                                      θ                    +                                                                  11                        6                                            ⁢                                              xe2x80x83                                            ⁢                      π                                                        )                                                                                    sin                ⁢                                  xe2x80x83                                ⁢                                  (                                      θ                    +                                                                  1                        2                                            ⁢                                              xe2x80x83                                            ⁢                      π                                                        )                                                                    ]              ⁡          [                                    Iu                                                Iv                              ]      
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel in response to the rotational position of the brushless motor.
It is preferable that the mentioned d-q axis voltage command section adds a signal, to which proportionals control and integral control have been applied, to the mentioned current deviation values.
It is preferable that a computing expression for performing the d-q coordinate inverse transformation on the basis of the mentioned detected current signal and the mentioned electrical angle signal is stored in the mentioned d-q coordinate inverse transformation section.
It is preferable that a following mathematical expression is used as the mentioned computing expression:             [                                                  Vu              *                                                                          Vv              *                                          ]        =                                        2            3                          ⁡                  [                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        1                          2                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                  θ                                                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        11                          6                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        4                          3                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                ]                    ⁡              [                                                            Vd                *                                                                                        Vq                *                                                    ]                        Vw      *        =                  -                  Vu          *                    -              Vv        *            
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel in response to the rotational position of the brushless motor.
An electric power steering control method according to the invention arranged to control a drive current of a brushless motor that generates a steering assist force on the basis of a steering torque acting on a steering shaft and a vehicle speed, comprises:
a first step of computing a d- and q-axis current command value for generating a steering assist force in accordance with a predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed;
a second step of performing a d-q coordinate transformation on the basis of a detection current signal in response to a three-phase AC current to be inputted to the mentioned brushless motor and an electrical angle signal in response to a rotational position of the mentioned brushless motor, thereby obtaining ad- and q-axis detection current value;
a third step of computing a d- and q- axis voltage command value in response to a deviation between the mentioned d- and q-axis current command value and the mentioned d- and q- axis detection current value;
a fourth step of performing a d-q coordinate inverse transformation on the basis of the mentioned d- and q-axis voltage command value and the mentioned electrical angle signal, thereby computing a three-phase voltage command value; and
a fifth step of generating a PWM waveform for PWM drive of the mentioned brushless motor in response to the mentioned three-phase voltage command value.
Thus, the method is arranged so that the brushless motor is used as the motor for generating a steering assist force, and the current control is performed on the d-q coordinate axis where the electric current can be treated in dc amount even when the motor is driven, and as a result, it is possible to achieve a reliable electric power steering control system in which error in torque control and transient vibration are both small even when the steering wheel is turned sharply.
It is preferable that, in the mentioned first step, the q-axis current command value for generating the steering assist force in response to the predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed is computed, and zero is set as the d-axis current command value.
It is preferable that, in the mentioned second step, the d-q coordinate transformation on the basis of the mentioned detected current signal and the mentioned electrical angle signal is performed in accordance with a computing expression preliminarily stored.
It is preferable that a following mathematical expression is used as the mentioned computing expression:             [                                                  Vu              *                                                                          Vv              *                                          ]        =                                        2            3                          ⁡                  [                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        1                          2                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                  θ                                                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        11                          6                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        4                          3                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                ]                    ⁡              [                                                            Vd                *                                                                                        Vq                *                                                    ]                        Vw      *        =                  -                  Vu          *                    -              Vv        *            
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel in response to the rotational position of the brushless motor.
It is preferable that, in the mentioned fourth step, the d-q coordinate inverse transformation on the basis of the mentioned d-axis and q-axis voltage command values and the mentioned electrical angle signal is performed in accordance with a computing expression preliminarily stored.
It is preferable that, in the mentioned fourth step, a following mathematical expression is used as the mentioned computing expression:             [                                                  Vu              *                                                                          Vv              *                                          ]        =                                        2            3                          ⁡                  [                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        1                          2                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                  θ                                                                                                      sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        11                          6                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                                                    -                    sin                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                          θ                      +                                                                        4                          3                                                ⁢                                                  xe2x80x83                                                ⁢                        π                                                              )                                                                                ]                    ⁡              [                                                            Vd                *                                                                                        Vq                *                                                    ]                        Vw      *        =                  -                  Vu          *                    -              Vv        *            
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel in response to the rotational position of the brushless motor.
It is preferable that, the motor control process according to the mentioned second to fifth steps is performed as an interruption of a cycle shorter than that of the d-q axis current command computing process according to the foregoing first step.